It is a known fact that nitrogen oxides are one of the most important pollutants produced during combustion processes, and are found in exhaust fumes released into the environment by steam generators, thermoelectric power stations and other industrial installations in which fossil fuels are burned. The formation of nitrogen oxides is due in part to the presence of N-compounds in the fuel (chemical NOx) and in part to the atmospheric nitrogen in the combustion air (thermal NOx). The NOx formation during the combustion process is dependent on a number of parameters, among which the main ones are the flame temperature, the time the combustion gases remain in the high-temperature zone and the excess air. More specifically, formation of nitrogen oxides increases as the flame temperature increases, and can be minimised by keeping the peak values under control. A similar effect is produced when the fuel remains in the high-temperature zone for a short time and by a reducing atmosphere or an excess of fuel in the ignition area.
From what has been stated above it is clear that a combined control of the above mentioned operating parameters will allow the formation of nitrogen oxides during combustion to be limited.
In burners in which it is possible to burn solid fuel in powder form, for example coal dust, the geometry of the air-coal mixture injector nozzle is of primary importance for the control of combustion and therefore for the production of NOx. In particular it is necessary to create areas that are poor in oxygen and rich in coal in the ignition area, and furthermore the time that the fuel remains therein must be such as to minimise the production of NOx and unburned substances. Finally, the recirculation streams around the end of the nozzle must be controlled in order to avoid excessively high temperatures and local fusion phenomena, which would damage the nozzle irreversibly. In a solid fuel nozzle of a known type manufactured by Foster Wheeler Energy Corporation (see U.S. Pat. No. 5,347,937), tangential inlet for the air-coal mixture is provided to convey said mixture towards four or more ducts, whose cross-sections converge towards the combustion chamber and which are arranged in correspondence with the outlet into said chamber along an annulus coaxial with the primary air duct. In this way jets of air-coal mixture rich in coal dust are formed. Between two adjacent mixture ducts there is a duct with a diverging cross-section, through which air is injected into the ignition area, said air being taken up through slots formed on the external shell of the nozzle. Injection of the air through these ducts is necessary in order to prevent the temperature of the nozzle from reaching excessively high values, but it also increases the level of oxygen in the ignition area, with an adverse effect on the reduction of NOx and therefore on the performance of the nozzle.